This invention relates to phototransformation of fullerenes, i.e., photo-induced and photo-assisted reactions which involve fullerenes.
The discovery of stable fullerenes, or carbon cage molecules C.sub.n (n=60, 70, . . . ) by Smalley, Kroto and coworkers has led to a new class of carbon-based solids with unusual properties. At room temperature, the molecules in solid C.sub.60 are centered on lattice positions of a face centered cubic ("fcc") structure, and are observed in NMR experiments to be rapidly rotating about these lattice positions. This unusual behavior is consistent with the weak intermolecular van der Waals bonding and the nearly spherical character of the C.sub.60 molecule. A polymerized C.sub.60 network has recently been reported. In one study, the C.sub.60 molecules were found to be linked together by a hydrocarbon bridge formed in the reaction of C.sub.60 in solution (toluene) with di-radical xylylene. Photoconductivity experiments of mixtures of C.sub.60 and polystyrene suggest that C.sub.60 attaches to polystyrene polymer.
The reactivity of C.sub.60 with oxygen has been known to occur in O.sub.2 -saturated benzene in the presence of ultraviolet light. The reaction of dioxygen (O.sub.2) with solid C.sub.60 was first noted in XPS studies of thin solid C.sub.60 films sublimed onto GaAs substrates. In these experiments, photo-assisted reactions were observed which involved the surface formation of CO, CO.sub.2, carbonyl groups and possibly amorphous carbon. The oxidation of C.sub.60 in oxygen-saturated benzene to form epoxide was subsequently reported. Later, it was shown that a reaction of solid C.sub.60 with dimethyldioxirane also leads to the formation of the C.sub.60 epoxide and a C.sub.60 -1,3-dioxolane derivative. Thermal oxidation of C.sub.60 powder in the presence of oxygen by infrared emission spectroscopy was also observed. Furthermore, it has also been demonstrated that a photo-induced reaction of C.sub.60 with excited molecular oxygen .sup.1 O.sub.2 leads to an incision in C.sub.60 cage due to the formation of oxides. In the case of higher fullerenes such as C.sub.70 , it has been shown that a photochemical reaction of C.sub.70 with oxygen leads to the formation of C.sub.70 O with oxa-bridged annulene structure. Finally, an enhancement in the resistivity (by a factor of 10.sup.4) of pristine C.sub.60 that was caused due to the thermal absorption of oxygen in single crystal C.sub.60 was recently reported.
Based on the high resolution .sup.13 C NMR spectrum of fcc C.sub.60 exposed to 1 kbar oxygen for 1.75 h at room temperature, it was reported that the O.sub.2 molecules occupy the octahedral interstitial sites of the fcc C.sub.60 lattice, with 8% of the octahedral sites being occupied by O.sub.2. Also, it was shown from the singlet oxygen photoluminescence spectrum of oxygen intercalated C.sub.60 crystal that oxygen is present as O.sub.2, rather than as atomic oxygen bonded to the C.sub.60 molecules. In the presence of photo-excitation, C.sub.60 is known to be 100% efficient in the generation of the highly reactive and unstable singlet (.sup.1 O.sub.2) oxygen.